The present invention relates to a superconducting magnet device to be used in a crystal pulling device for generating a crystal for a semiconductor device, in particular, to a superconducting magnet device for a crystal pulling device capable of reducing the leakage magnetic field.
In pulling a crystal from a molten semiconductor crystal material with a pulling device for crystals for a semiconductor, a superconducting magnet device is used as a static magnetic field generating device for preventing the movement by the heat convection by applying the static magnetic field to the molten semiconductor crystal material.
FIG. 1 is a perspective view showing a configuration of a conventional superconducting magnet device for a crystal pulling device. FIG. 2 is a cross-sectional view viewed from the direction shown by the arrows II--II of FIG. 1. As shown in FIG. 1, the superconducting magnet device is provided such that two superconducting coils 2a, 2b are arranged facing with each other in a C-shaped cryostat 1, and the magnetic field direction 3 generated by the superconducting coils 2a, 2b is orthogonal to the crystal pulling direction 4.
Furthermore, as shown in FIG. 2, the cryostat 1 has the superconducting coil 2a in a helium container 40 so that the coil 2a is soaked in a coolant 41 such as liquid helium, and the like. Moreover, a radiation shield 42 is provided outside the helium container 40, and the entirety of the helium container 40 and the radiation shield 42 is stored in a vessel 43 so as to comprise the cryostat as a whole.
Although the case provided with one radiation shield 42 is shown in the above-mentioned example, double radiation shields can be provided as needed.
Furthermore, a system where the superconducting coils 2a, 2b in the helium container 40 are cooled down to the superconducting state with a freezer without using a coolant such as liquid helium, and the like, is also be used. In this case, the helium container and the coolant shown in FIG. 2 are not provided.
This is an example of a magnetic device where a crystal pulling device is provided in the bore 5 at the center part of the C-shaped cryostat 1 for applying the horizontal magnetic field to the crystal pulling device.
FIG. 3 shows another configuration of a conventional superconducting magnetic device for a crystal pulling device. The superconducting magnet device is provided such that two superconducting coils 2a, 2b are provided facing with each other, and the magnetic field direction 3 generated by the superconducting coils 2a, 2b is orthogonal to the crystal pulling direction 4 as in FIG. 1 in a double-cylindrical cryostat 11 comprising the inner cylinder and the outer cylinder.
This is an example of a magnetic device where a crystal pulling device is provided in the bore 12, which is a space of room temperature, inside the inner cylinder of the cryostat 11 for applying the horizontal magnetic field to the crystal pulling device.
FIG. 4 shows still another configuration of a conventional superconducting magnet device for a crystal pulling device. The superconducting magnet device is provided such that two superconducting coils 2a, 2b are provided facing with each other with an optional distance therebetween with respect to the axial direction of a cryostat 31 in the double-cylindrical cryostat 31 so that magnetic fields 30a, 30b are generated in the direction the coils 2a, 2b are opposed so as to generate the cusp magnetic field. In this case, the crystal pulling device is disposed in the bore 32, and the crystal pulling direction 4 is as shown in the figure. Since the magnetic field becomes zero on the line 6 in the figure in the cusp magnetic field, it is said that a high quality crystal compared with the cases of FIGS. 1 and 3 can be obtained by pulling the crystal on the line.
Since a nonmagnetic material such as stainless steel and aluminum has been used as the vessel of the cryostat in the above-mentioned conventional superconducting magnet devices for a crystal pulling device, the magnetic field generated from the superconducting coils is leaked to the outside of the magnet so as to affect a motor or component members made from a magnetic substance existing in the vicinity of the vessel. In this case, since the magnetic substance and the superconducting coils are attracted with each other and the attraction force is proportional with the volume, the weight and the magnetic field gradient of the magnetic substance, the larger the magnetic component member or the magnet are, the larger the attraction force becomes accordingly.
Therefore, it involves a problem in that the component members should be made from a nonmagnetic material, which is expensive, or the cross-section of the support in the cryostat for supporting the coils should be large, and thus the heat penetration amount of the cryostat becomes large.
Furthermore, if a motor or a measurement device easily affected by the magnetic field exists in the vicinity of the cryostat, it may lead to the malfunction of the devices.
Moreover, since the superconducting magnets adjacent to each other are affected by the leakage magnetic fields of each other if the leakage magnetic field is large so as to disturb the magnetic field strength or the magnetic field distribution necessary in the bore, the installation space of the adjacent magnets and the crystal pulling device should be wide, and thus the installation space cannot be used advantageously.
An object of the present invention is to provide a superconducting magnet device for a crystal pulling device, capable of reducing the leakage magnetic field at a comparatively low cost, and optimum for achieving the small size and the light weight.